Flexible electroluminescent device and light transmissive electrically conductive electrode material therefor



April 18, 1967 M. s. JAI-FE ETAL 3,315,111 FLEXIBLE ELECTROLUMINESGENT DEVICE AND LIGHT TRANSMISSIVE ELECTRICALLY CONDUCTIVE ELECTRODE MATERIAL THEREFOR Original Filed April 20. 1962 lm/erwfors Marg 5. darfdle Emmer G. Fvidrich wwf@ Their ATi-@They United States Patent O FLEXIBLE ELECTRULUMINESCENT DEVICE AND LIGHT TRANSMISSIVE ELECTRICALLY (N DUCTIVE ELECTRODE MATERIAL THEREFOR Mary S. Jaffe, Cleveland Heights, and Elmer G. Fridrich,

South Euclid, Ohio, assignors to General Electric Company, a corporation of' New York Continuation of application Ser. No. 189,095, Apr. 20, 1962. This application June 9, 1966, Ser. No. 556,837 9 Claims. (Cl. S13- 108) This application is a continuation of Ser. No. 189,095, filed Apr. 20, 1962, now abandoned.

This invention relates, in general, to electroconductive lacquer or film compositions of light-transmitting character and to the manufacture thereof, and more particularly to an electrode comprised of such electroconductive lacquer and to an electroluminescent cell or lamp equipped with such an electrode.

As known at present, electroluminescent cells or lamps, sometimes referred to as luminous capacitors, comprise in general a layer of an electroluminescent or field-responsive phosphor sandwiched between a pair of electrically conducting or electrode layers at least one of which is light-transmitting. When an alternating current of sufficient potential is impressed between the electrodes, the phosphor material is excited to luminescence and the resulting light is emitted through the light-transmitting electrode layer.

Electroluminescent cells or lamps in general use at present are either of the so-called rigid type having one or more vitreous or ceramic layers of inflexible character, or they are of the flexible type such as disclosed and claimed in U.S. Patent 2,774,004 to laffe, dated Dec. l1, 1956, in which all the component elements of the cell including the light-transmitting electrode layer are essentially of flexible character. The flexible type electroluminescent cells or lamps are, of course, advantageous not only because of their light weight and dimensionally thin character, but also because of their comparative nonfragility and suitability for forming into odd shapes and patterns.

The light-transmitting electrode layer most commonly employed heretofore in flexible type electroluminescent cells or lamps has consisted of electrically conductive glass paper such as, for example, that disclosed in Jaffe Patent 2,849,339, dated Aug. 26, 1958, and consisting of very fine glass brils compacted into paper-like sheet material andcoated with an electrically conductive refractory oxide or compound such as the heatdecomposi tion product of indium basic trifluoroacetate, the electrically conductive coating also serving to bond the glass fibrils together. Such electrically conductive glass paper is, however, very brittle mechanically. From the standpoint of the operative effectiveness of the electrically conductive glass paper as an electrode for an electroluminescent cell or lamp, this brittle characteristic of the conductive glass paper is of great advantage in that it causes the conductive glass paper to become crushed to a fine powder by the pressure attending the lamination of the electroluminescent cell or lamp. As a consequence, the crushed glass paper particles then conform intimately to the comparatively irregular, i.e., gritty, surface of the phosphor layer of the electroluminescent cell, thereby bringing the electrical field present during lamp operation closely down to the phosphor surface and so assuring application of the maximum available voltage across the phosphor layer with resulting maximum production of light therefrom. From a manufacturing standpoint, however, the brittle character of the conductive glass paper is disadvantageous not only because of the appreciable 3,3 15,1 l l Patented Apr. 18, 1967 susceptibility of the conductive glass paper to breakage during the handling and cutting thereof preparatory to its incorporation into an electroluminescent cell or lamp, with resulting high production rejects or shrinkage as it is sometimes called, but also because of the increase in the electrical resistance of the conductive glass sheet which occurs when it becomes crushed into a powder during the cell laminating operation. This increase in the electrical resistance of the conductive glass sheet is undesirable especially for large area cells or lamps, or for cells or lamps which are to be operated at high frequency, since the series resistance of the conductive glass layer reduces the voltage available for the phosphor layer. As a result, the lamp becomes dimmer at points progressively of less and markedly decreased susceptibility to production defects than prior flexible type electroluminescent cells and which possesses at least substantially comparable lighting performance characteristics.

Another object of our invention is to provide a flexible type electroluminescent cell having an electrode consisting solely of a layer of a light-transmitting electrically conductive lacquer film.

Still another object of our invention is to provide a flexible type light-transmitting electrode for electrolumiprior electrodes of such type.

A still further object of our invention is to provide a flexible type light-transmitting electrode for electroluminescent cells and similar devices which is of composite character comprised of a layer of light-transmitting electrically conductive glass paper and a layer of a light transmitting electrically conductive lacquer film bonded to a surface of the conductive glass paper to serve as a conductivity aid therefor.

Another object of our invention is to provide a lighttransmitting electrically conductive lacquer film composition suitable for use as a lightatransmitting electrode of an electroluminescent cell or similar device.

Briefly stated, in accordance with one aspect of our invention, an electroluminescent cell or lamp is provided with a light-transmitting electrode comprised at least in part of a layer of an electrically conductive lacquer film comprising a substantially transparent organic plastic having light-transmitting electrically conductive particulate material dispersed therein consisting either of translucent electrically conductive powder particles such as certain metal oxides for instance, or fibrous particles of glass or other suitable inert material provided With a light-transmitting coating of electrically conductive ma-` terial such as indium oxide, for example. The lighttransmitting electrode of the electroluminescent cell may be composed in its entirety of a layer of the electrically conductive lacquer film, or it may be of composite character comprised of a light-transmitting electrically conductive web of reticulate character such as electrically conductive paper or a fine, open-structure type metallic web, mesh, grid or perforated sheet, and a layer of a light-transmitting electrically conductive lacquer lm disposed between the electrically conductive web and the phosphor layer of the cell to serve as a conductivity aid for the reticulate web.

Further objects and advantages of our invention will appear from the following detailed description of species thereof and from the accompanying drawing.

In the darwing, FIG. 1 is a pictorial view of a flexible electroluminescent cell according to the invention, with the various constituent layers thereof delaminated or peeled open at one corner to show the internal construction of the cell.

FIG. 2 is a pictorial view similar to FIG. 1 of a modied form of flexible electroluminescent cell according to the invention.

FIG. 3 is a fragmentary side elevation, on a greatly enlarged scale, of one form of light-transmitting lacquer film electrode according to the invention for use in the lamps of FIGS. l and 2.

FIG. 4 is a fragmentary side elevation, on a greatly enlarged scale, of another form of light-transmitting lacquer lm electrode according to the invention for use in the lamps of FIGS. 1 and 2, and

FIG. 5 is a fragmentary sectional view, on a greatly enlarged scale, of a prelaminate of organic plastic sheet material and electrically conductive glass paper coated with an electrically conductive lacquer film according to the invention, for use as the light-transmitting electrode of an electroluminescent lamp of the type illustrated in FIG. 2.

Referring to the drawing, the electroluminescent cell or lamp 1 there shown consists of a flexible panel comprised of an inner electrically active -cell portion or assembly 2 sealed within a substantially moisture-impervious outer encapslating envelope portion 3. The cell 1, which in the particular case illustrated is of rectangular shape, may be energized by applying a suitable potential such as an alternating voltage, for example, 120 volts 60 cycles A.C., to ribbon-type electrical conductors 4 and 5 projecting laterally from the edge of the outer envelope 3. The conductors 4 and 5 are preferably formed of relatively ne mesh wire cloth, for example, 200 to 300 mesh, of suitable electrically conductive material such as copper or Phosphor bronze, the ribbon conductors being connected to respective ones of the lamp electrodes. The outer envelope 3 is composed of sheets 6 and 7 of suitable material which seals together under heat and pressure. Sheets 6 and 7 overreach the marginal edges of the electrically active cell portion 2 and are sealed together along their margins so as to completely enclose the cell portion 2. The materials selected for the encapsulating envelope 3 are preferably tough and stable in addition to exhibiting light-transmitting qualities and high impermeability to moisture, and further they are preferably flexible in nature. Among the materials Which may be satisfactorily employed for this purpose are polyethylene, polytetrauor-oethylene, polychlorotriiluoroethylene, polystyrene, methyl methacrylate, polyvinylidine chloride, polyvinyl chloride, polycarbonate materials such as, for example, the reaction products of diphenylcarbonate and Bisphenol A and polyethylene terephthalate. The materials preferably employed for such purpose, however, consist either of polychlorotrifluoroethylene lm, known as Kel F, of approximately 0.005 inch thickness, or of resin-impregnated mica mat such as that disclosed and claimed in copending U.S. application Ser. No. 118,113, Levetan, filed June 19, 1961, and now abandoned, assigned to the same assignee as the present invention.

The electrically active inner portion or assembly 2 of the electroluminescent cell or lamp 1, i.e., the lightproducing components thereof, is constituted by a flexible panel assembly essentially comprised of a phosphor layer 8 sandwiched between a pair of electrode layers 9 and 10 at least one of which, eng., the front electrode 10, is of light-transmitting character having a transmittance of at least 60%, and is of the type comprising our invention, Except for the front electrode component 10 thereof, the electrically active cell portion 2 may consist of any of The insulating or barrier F 1 is the lightatransmitting the known types of flexible electroluminescent cell assemblies which are of non-fragile character and light in weight. Preferably, however, it is of the general form disclosed and claimed in U.S. Patent 2,945,976, Fridrich et al., dated Iuly 19, 1960 and comprising a rectangular sheet of thin metal foil 9, for instance full-soft aluminum of around 0.0022 inch thickness, coated with a thin insi1- lating or barrier layer 11 of high dielectric constant material which is overcoated with a thin light-producing layer 8 of an electroluminescent phosphor dispersed in a dielectric material. The aluminum foil sheet 9 constitutes the back electrode layer of the lamp and is placed over the lowermost sheet member 6 of the encapsulating envelope 3 leaving a clear margin all around, as shown in FIG. l. layer 11, which suitably may be of a thickness of, for example, around 1 mil or so, may consist of barium titanate dispersed in an organic polymerio matrix of high dielectric constant such as cyanoethyl cellulose plasticizeo' with cyanoethyl phthalate as described and claimed in U.S. Patent 2,951,865, Jaffe et al., issued Sept. 6, 1960, and in US. Application Ser. No. 701,907,- Jaffe, filed Dec. 10, 1957 now US. Patent No. 3,238,407 and assigned to the same assignee as the present invention. Other suitable organic polymeric matrix materials for the barium titanate insulating layer 11 are cellulose nitrate, cyanoethyl starch, polyacrylates, methacrylates, polyvinyl. chloride, cellulose acetate, alkyd resins, epoxy contents, and polymers of triallyl cyanurate, to which may be added modifying substances or plasticizers such as camphor, dioctyl phthalate, tricresyl phosphate and similar materials. The electroluminescent phosphor layer 3, which likewise may suitably be of a thickness of, for example, around l. mil or so, may consist of any known electroluminescent phosphors such as zinc sulde-Zinc oxide, with suitable activators such as copper, chlorine and manganese,- like-1 wise dispersed in lan organic polymeric matrix such as that used in connection with the insulating or barrier layer 11i.- The barium titanate layer dispersed in a cyanoethyl cellulose solution may be applied to the aluminum foil 9 by spraying, or preferably through the use of a doctor blade, and then drying. The phosphor layer, likewise dispersed in a cyanoethyl cellulose solution, may then be applied over the barium titanate layer 11 in a similar manner.-

Overlying the phosphor layer S of the electrically ac-y tive portion or assembly 2 of the electroluminescent cell front electrode layer 10 of the; type 4comprising our invention, and over this electrode layer 10 there is preferably placed a desiccant layer 1f' of a suitable transparent plastic material which exhibits hydrophilic properties, i.e., has an affinity for water. As disclosed in copending U.S. application Ser. No. 80,613 of Devol et al., filed Ian. 4, 1961, now U.S. Patent No. 3,148,299 and assigned to the same assignee as the present invention, polyamide condensation products such as nylon 6,6 or nylon 6 such as that known as Caplene, have been found to be particularly effective as' hydrophilic materials for the plastic layer 12.

In manufacturing the electroluminescent cell or' lamp? 1, the electrically active portion 2 of the cell is placed beJ tween the two encapsulating sheets 6 and 7, with the two conductors 4 and S in respective contact with the metal foil back electrode 9 and the light-transmitting front electrode 10, and the stacked assembly then subjected to heat and pressure to laminate the cell components together. The laminating of the cell components may be performed in the manner, and by the iuse of a hydrostatic laminating press such as described and claimed in the aforesaid Fridrich et al. Patent 2,945,976 or in copending U.S. application Ser. No. 748,537, Fridrich, tiled July 14, 1958, now US. Patent No. 3,047,052 and assigned to the assignee of the present invention. As therein described, the stacked assembly of cell components is placed between the top and bottom platens of the hydrostatic press, beneath a conformable diaphragm separating the press platens, the conformable diaphragm being constituted of a exible gas-v impervious sheet material such as soft annealed aluminum foil or polyethylene terephthalate lm such as Mylar. Pressurized gas is admitted into the closed chamber of the laminating press over the diaphragm therein to exert hydrostatic pressure on the cell components, vacuum is applied under the diaphragm to remove any trapped gases or moisture from the space therebelow, and heat is then applied by suitable means to the stacked assembly of cell components to cause the outer plastic sheets 6 and 7 to soften and seal together at their margins so as to encapsulate the electrically active cell portion 2. During the laminating process, the conductors 4 and 5 Ibecome embedded in the plastic sheets 6,7 and are at the same time pressed into intimate contact with the back electrode layer or metal foil 9 and the light-transmitting front elec"- trode layer 10, respectively, so as to be in good electrical contact therewith.

In accordance with the invention, the light-transmitting electrode 10 of the elec-troluminescent cell or lamp 1 is comprised, at least in part, of a :relatively thin layer 13 (e.g., around 0.1-0.6 mil thick) of a light-transmitting electrically conductive lacquer lilm consisting essentially of a dispersion of electrically conductive or semi-conductive translucent particulate material 14 in a binder or matrix 15 of a substantially transparent soluble organic plastic having as high an electrical conductivity as possible. The layer 113 of electrically conductive lacquer may constitute the entire electrode 10, as in the form of the inven-tion shown in FIG. l, or it may constitute one of the component parts or layers of a composite electrode 16, as shown in FIG. 2, the other component layer 17 of which is constituted of a light-transmitting electrically conducting web of reticulate character such as fine wire cloth or tine perforated metal screen, or semi-conductor coated glass cloth or paper such as that ydisclosed in U.S.

Patent 2,774,004, laffe, dated Dec. ll, `1956. However, I

because of its greater ruggedness and resistance to breakage than such semi-conductor coated glass paper, the reticulate web 17 is preferably constituted of electrically conductive paper comprised of metallizcd micro-fibrils of glass dispersed among and supported by translucent iibers of non-conducting material such as glass, cellulose (rayon), cyanoethyl cellulose, Orion, nylon, or woodpulp bers such as employed in ordinary paper.

Where the light-transmitting electrode `10 is constituted in its entirety of a layer 13 of the electrically conductive lacquer, as in the form of the invention shown in FIG. l, then in such case the lacquer employed for the electrode 10 is one which, for the purposes of the invention, is of comparatively highly conductive character. In this connection, a highly conductive lacquer may be categorized as one having a specific impedance within the range of from 1 to 100 ohm-cm., for 60 cycle current at l amp/ cm2 current density. Stich highly conductive lacquers have a resistance of less than `10,00() ohms per square at im or layer thicknesses of less than 10 microns, which are typical thicknesses employed for the conductive lacquer layer `13.

In the case where the layer 13 of conductive lacquer constitutes, as in the form of the invention shown in FIG. 2, a part only of a composite multi-layered electrode 16 the other layer of which is composed of an electrically conductive web 17 of reticulate character such as ordinarily is, in itself, of comparatively highly conductive character, then in such case it suffices to employ for the lacquer layer 13 an electrically conductive lacquer of comparatively weakly conductive character, which may be categorized as one having a specic impedance above 1,000 ohm-cm. and ranging up to as high as the order of 1,000,000 ohm-cm., for 60 cycle current at -l amp/cm.2 current density. Such weakly conductive lacquers have a resistance of more than 50,000 ohms per square at the typical thicknesses (around 10 microns or less) normally employed for the conductive lacquer layer 13. No added benet in l-amp lighting performance or brightness is realized from the use, in conjunction with a comparatively highly conductive reticulate electrode layer 17 of a composite electrode 16 according to the invention as shown in FIG. 2, of an electrically conductive lacquer for the electrode layer 13 of either a strongly or intermediate degree of electrical conductivity, i.e., one having a specific impedance below approximately 1,000 ohms-cm., for 60 cycle current at a current density of l amp/cm.2 In fact, the use in such case of either a highly or an intermediate conductive lacquer for the electrode layer l13 act-ually has been found to be somewhat of a. 4disadvantage because of the attendant increased light absorption by such lacquers due to the-ir higher volume loading with translucent light-absorbing electrically conductive particulate material 14, the increased lightabsorption by the conductive lacquer layer 13 resulting, in turn, in diminished lamp brightness.

For the purposes of the invention, the organic matrix material or binder component 15 employe-d for the electrically conductive lacquer film 13 should be one that is substantially transparent, and preferably entirely colorless after processing into the lamp or cell 1, in order to thereby minimize absorption of the light: generated by the phosphor layer 8. In addition, the organic matrix material 1S should have as high an electrical conductivity as possible, and should be soluble in common solvents which evaporate readily and which do not adversely affeet the phosphor coating. Moreover, to enable convenient coating application of the conductive lacquer onto substrates by conventional machine coating devices such as doctor blade coaters, the organic plastic matrix material or binder l15 preferably should be one whose solutions are fairly viscous, i.e., within the range of from 20 to poises an-d preferably around 30 to 50 poises, at fairly low weight percent of solids, c g., around l0 to 15 weight percent of solids. In this connection, the adaptability of the conductive lacquer mix to machine coating requires that the lacquer mix possess rheological properties of somewhat conflicting character. On the one hand, a rather low plastic or solids-to-solvent ratio is necessary in order to permit reasonable and easily controllable wet coating thicknesses of, for example, around 2 mils, and at the same time obtain the desired relatively thin (e.g., around 0.2 to 0.4 mil thickness) rinal or dried coatings. Such a wet-to-dry thickness ratio therefore requires that about a 10 to 15% plastic solution be used. On the other hand, the viscosity of the conductive lacquer mix must be maintained at a level high enough, eg., above 20 poises and -up to 150 poises, for convenient coating by machine coating devices. While conductive lacquer mixes having viscosities of less than 20 poises can be coated by hand, or in small machine-coated lots, they are inconveniently f iuid and, when machine-coated, they tend to flow out around the doctor blade of the coating machine and create a wide, slow-drying beading, thus not only wasting coating material but also causing drying 'diiculties Since the addition of the conductive particulate material or filler 14 to the plastic solution does not greatly `atte-ct the viscosity of the conductive lacquer mix, the pure plastic solution therefore should have the desired viscosity of between 20 to 150 poises at a concentration of about 10 to 15% by weight of solids or plastic in solvent. For spray application of the conductive lacquer layer `13, the lacquer may be less viscous than that specified above as being preferred for machine coating application. The lower viscosity may be achieved by employing a greater amount of dil-uent solvent, or by using an organic plastic of lower intrinsic viscosity. The organic matrix material 15 employed for the electrically conductive lacquer 13 should also adhere well, when thoroughly dry, to the phosphor layer 8 of the electroluminescent cell or lamp 1 and, where used in conjunction with a layer of electrically conductive glass paper 1-7 to form a composite electrode 16 as in FIG. 2, it should be suciently plastic to How into and bond well to the conductive glass paper fibrils.

Of all the various requirements mentioned `above for the organic plastic matrix material of the conductive lacquer `13, the requirement that it possess as high an electrical conductivity as possible is the most difficult to meet because presently yavailable polymers are essentially good insulators. `inasmuch as substantially all electroluminescent lamps as made at present are alternating current operating devices, a matrix material 15 having a high alternating current admittance is therefore highly desirable since Stich a high alternating current admittance characteristic favors electrical conductivity. Accordingly, the organic plastic materials preferably employed for th-e matrix or binder material 15 of the electrically conductive lacquer 13 should have a relatively high permittivity or dielectric constant since a high dielectric constant irnplies a high alternating current admittance. Organic polymeric materials which possess the required properties mentioned above to various degrees and which therefore may be employed for the matrix or binder component 15 of the conductive lacquer 13 are polystyrene, cellulose acetate butyrate, polymethacrylates such as, for example, polymethylmethacrylate, and polyamides such as nylon polyamides rand polymers and copolymers of acrylamide or other vinyl -amides such as, for example, that commercially known as Cyanamer made by American Cyanamid Company. However, because of their outstanding electrical and theological properties, by far the best organic polymeric materials suitable for use as the matrix or binder component 15 of the conductive lacquer 1113 are cyanoethyl starch or cyanoethyl cellulose, or mixtures thereof, plasticized with cyanoethyl phthalate or cyanoethyl sucrose. Thus, the organic plastic matrix 15 may comprise the same cyanoethyl cellulosecyanoethyl phthalate plasticdescribed hereinbefore as the preferred matrix material for the phosphor layer 8 and the barium titanate insulating layer 11. Such cyanoethyl starch and cyanoethyl cellulose matrix materials plasticized with cyanoethyl phthalate or cyanoethyl sucrose possess a remarkably high permittivity or dielectric constant of at least 25 or s0, which is several times higher than that of any of the other known organic polymeric materials referred to above as being suitable for use as the matrix material 15. Due to their considerably higher dielectric constant, therefore, such cyanoethyl starch yand cyanoethyl cellulose matrix materials 15 plasticized with cyanoethyl phthalate orcyanoethyl sucrose will possess `a higher alternating current admittance and thus produce a more highly conductive lacquer composition 13. The organic plastic matrix materials 15 which are comprised of cyanoethyl cellulose -or cyanoethyl starch plasticized with cyanoethyl sucrose are not our invention but instead are the invention of, and form part of the subject matter described and claimed in the copending United States `application of Robert V. Levetan, Ser. No. 247,648, filed Dec. 27, 1962, and assigned to the same assignee as the present invention.

The inorganic electrically conductive particulate material employed as the filler or pigment component 14 of the electrically conductive lacquer lm 13 constitutes the principal source of electrical conductivity, therefor, since the organic plastics suitable for use as the matrix material 15 all are fairly insulating in character. The main requirements for the particulate lfiller material 14 are high electrical conductivity coupled with low light-absorption (or high light transmission). This last mentioned requirement of low light absorption rules out any form of -finely ydivided metal as the conducting filler material 14 inasmuch as the electrical conductivity of the lacquer composition may be expected to increase no more than linearly with volume loading of the electrical-ly conductive filler material 14 whereas the optical absorption of the lacquer composition will increase at least exponentially, in the case where finely divided metals are employed as the filler material 14., due to the high optical absorption coefficient of metals. The concentration or volume loado CD ing of the electrically conductive particulate filler or pigment material 14 in the conductive lacquer film 13 necessarily must be a compromise between electrical conductivity and optical transmissivity of the lacquer composition. Thus, for high optical transmission the conductive lacquer electrode 13 should be in the form of a very thin laye-r with relatively low conductive pigment loading, while high electrical conductivity calls for a thick layer with high conductive pigment loading. For the purposes of the invention, however, it has been determined that, with the range of thicknesses (i.e., from approximately 2 to l5 microns) normally employed for the conductive lacquer film layer 13 in electroluminescent lamps according to the invention, the concentration of the electrically conductive ller or pigment material 14 in the conductive lacquer composition according to the invention Ishould amount to between 10 to 50%, by volume, of the final, dried lacquer film composition. Below approxi- -mately 10%, by volume, loading 4of fil-ler material 14, the `specic impedance of the dried conductive lacquer film composition becomes too high (i.e., in excess of 1,000,000 ohm-cm., for 60 cycle current at l amp/ cm.2 current density) for satisfactory use as an electrode layer 13 of an electroluminescent lamp 1. On the other hand, more than approximately 50%, by volume, loading of filler material 1.4 in the final dried lacquer composition produces too much light scattering and light absorption to render it satisfactory for use as an electrode layer I3 of an electroluminescent lamp. Within the limits of concentration Aspecified above for the filler material 14, the preferred loading or concentration range therefor is approximately 20 to 30%, by volume, of the final, dried lacquer composition.

The electrically conductive filler material 14 may be in the form of a single solid phase consisting of particles of material which itself is of electrically conductive character, or it may be in the form of a coated substrate system consisting of inert transparent substrate particles coated with a thin light-transmitting film o-f electrically conductive material. Also, the individual particles of the electrically conductive filler material 114 may be of any physical shape such as, for example, in the form of grains .as shown in FIG. 3, or in the form of flakes or fibrils. However, the preferred shape for the electrically conductive particles 14 is acicular or needle-like as shown in FIG. 4, such a particle shape yfavoring the efcient spreading of the electrical field laterally across the surface of the conductive lacquer layer 13 of the electroluminescent lamp 1, with a minimum of light scattering and light absorption, whereby improved lamp brightness and uniformity of illumination is obtained.

Among the various single solid phase type of materials that may be suitably employed for the electrically conductive particulate filler material 14, there are a considerable number of stable, homogeneous metal oxides of white or light-colored character which exhibit appreciable electrical conductivity when properly prepared. These oxides are all of metals which can exist in more than one valence state and which can be doped with irnpurity atoms to induce higher electrical conductivity according to well known principles. `in addition, almost all these metal oxides are known to be n-type conductors; hence, the doping agents will, in general, be metal atoms of higher valency as substitutional impurities, possibly along with partial replacement of oxygen by halogen. The preferred metal oxide for such purpose is indium oxide such as the commercial grade .indium oxide made by Indium Corporation of America. The indium oxide powder may be doped with a few percent of tin and subjected to a suitable firing, as by 'being heated in an open quartz or alumina. dish for a period of five to twenty minutes at a temperature in excess of 500 C. but below the melting point of the oxide. Other refractory metal oxides suitable for use as the electrically conductive particulate filler material 14 are the oxides of Zinc, cadmium,

trically conductive iiller material -14 not only because of the fact that it is a cheap raw material but also because of its white and therefore high light-transmissive body color, and also because its crystals under some circumstances grow in needles which, as stated previously, favors the eliicient spreading of the electrical field across the surface of the conductive lacquer layer 13, with a minimum of light scattering and resultant loss of light by absorption in the phosphor layer.

The use of electrically conductive particulate material 14 in the form of coated substrate systems consisting of inert transparent substrate particles coated with a thin light-transmitting lm of electrically conductive material such as a metal oxide, has the advantage, over the single solid-phase type conductive filler material, that the substrate particles may be of a shape suitable for eiiicient spreading of the electrical field (eg, acicular or needlelike shape as shown in FIG. 4), independent of the crystal habit of the oxide coating. Moreover, where the substrate particles are of very smooth surface character so as to give rise to minimal light scattering, and where the electrically conductive coating on the substrate particles is, in addition, very thin so as to minimize the light absorption thereby, important improvements then can be achieved in the optical transmission of the conductive lacquer film 13. The preferred form of coated substrate type particulate material employed for the electrically conductive filler or pigment 14 of the conductive lacquer -iilm 13 is composed of electrically conductive glass paper which has been crushed to a fine powdered state. The electrically conductive glass paper preferably employed for such purpose is of the general type, and may be prepared in the manner described in the previously mentioned Jaffe Patents 2,774,004 and 2,849,339, such conductive glass paper consisting of very fine glass fibrils or microiibers which are compacted into paper-like sheet material and which are coated with and supported by an electrically conductive metal oxide or other compound, preferably indium oxide. The glass fibers of which present commercially available glass papers are composed generally are 'between 0&1 to 3 microns in thickness, with the vast majority of them between approximately =1 to 2 microns in thickness. As disclosed in the said Jaffe patents, the preferred method of making the glass paper electrically conductive consists in dipping the glass paper in a solution of a metal salt which upon subsequent drying and baking forms a conductive coating. The preferred coating solution for such purpose consists of indium basic trifiuoroacetate In( OH) (GF3COO)2 with stannic chloride (SnCl4) dissolved in an organic solvent such as ethylene glycol monoethyl ether acetate (Cellosolve acetate of Carbide and Carbon Chemicals Corporation). The glass paper, after being dipped in such a preferred coating solution, is then dried and fired at an elevated ternperature in excess of 250 C. suiiicient to decompose the indium basic triiiuoroacetate to a stable water-insolubl transparent and electrically conducting compound of indium, believed to be an indium oxide of poorly developed crystallinity. The electrically conductive glass paper thus lformed is then milled in a ball mill until it becomes crushed into a fine powder comprised of electrically conductive translucent particles 14. When the electrically conductive glass paper is thus crushed and subsequently mixed with an organic plastic solution such as described above to form an electrically conductive lacquer according to the invention, and the conductive lacquer then cast into Afilms as shown in FIG. 4, the resultant dried lacquer film is a darker gray-green than the original starting (i.e., uncrushed) electrically conductive glass paper, and even in thin iilms absorbs some light. However, if ythe electrically conductive glass paper is first heated a short time in air to an elevated temperature of about 600 C., it turns to a light yellow color, corresponding to the body color of indium oxide. While such overheated conductive glass paper is less conductive in sh'eetform than the normally lightly (lower temperature) Ifired greenishwhite paper, nevertheless it forms, upon crushing, a powder which apparently is no less conductive than the crushed greenish-white conductive glass paper and with much better optical transmission in lacquer film form and more pleasing in color. For such reason, it is preferred to so preliminarily heat treat the electrically conductive `glass paper although it is entirely suitable for use for the particulate filler material 14 without any such heat treatment.

Other coated substrate materials that may be employed for the electrically conductive particulate filler material 14 of the conductive lacquer film 13 may consist of a transparent inert substrate material of either acicular shaped particles such as chopped glass roving or needlelike clays such as attapulgite, or of plate-shaped particles such as glass iiake, mica flake, and kaolinite-type clays, which are coated with a thin light-transmitting electrically conductive film such as indium oxide, for instance. While the electrically conductive coating could be applied to such finely divided substrate material in a manner similar to that described above for making glass paper conductive, i.e., by dipping the nely divided substrate material into a metal-organic coating solution followed by the drying and baking of the coated substrate material, a more suitable Way of accomplishing such object is by the use of the so-called vapor-phase technique wherein the iinely divided substrate material is contacted, while in a heated state, with the vapors of a metal salt such as tin tetrachloride or indium trichloride. For such purpose, the finely divided substrate material could be held at the required high temperature in a fluidized bed, and treated with the metal salt vapors; or the substrate material could be sifted down through a hot zone while the reactive metal salt vapors are percolated upward through and into contact with the sifted substrate material.

In preparing an electrically conductive lacquer according to the invention, the electrically conductive material employed for the ller 14, Whether of the single solidphase or of the coated substrate type, ordinarily must be tirst milled in a ball mill in order to eliminate large fibers or particles therefrom. This is so because of the fact that the presence of any such large iibers or particles of conductive filler material 14 in the conductive lacquer iilm layer 13 not only causes body arcing in the electroluminescent lamp 1 as a result of their penetrating the phosphor layer 8 (and the insulating or barrier layer 11 as Well where used) during the lamination ofthe lamp, but they also cause edge arcing in the case of those electroluminescent lamps 1 which are formed from cut stock comprised of the back electrode 9 coated with the phosphor layer 8 and overcoated with the conductive lacquer layer 13. This requirement for adequate milling of the electrically conductive material 14 is especially true not only in the case of the above described-electrically conductive glass paper because of its needle-like form when crushed, but also in the case of commercial grade indium oxide powder which ordinarily is coarse enough to cause such lbody and edge arcing in the electroluminescent lamp. To insure against any such arcing, therefore, in the finished electroluminescent lamp 1, it has been determined that the electrically conductive particulate material employed as the filler material 14 for the conductive lacquer 13 should have, in the case where it is in powder form as shown in FIG. 3, a particle size no greater than about 10 microns, with a preferred particle size of around 4 microns and under, while in the case where the particulate material 14 is in the form of flakes, or short fibers such as is characteristic of crushed conductive glass paper as shown in FIG. 4, the individual particles should then have a iiber or flake length no greater than around onehalf the combined thickness of the phosphor layer 8 and the insulating barrier layer 11, or the thickness of the phosphor layer alone in the case where no insulating layer 1 1 is employed in the lamp. In this connection, since the combined phosphor and barrier layers customarily employed in electroluminescent lamps ordinarily have thicknesses of at least around 50 microns, such fibrous or flaketype conductive particles 14 therefore should not be any longer than 20 to 25 microns, i.e., around 1 mil or so.

In addition to insuring against the occurrence of body and edge arcing in the electroluminescent lamp 1, the particle size reduction produced by the ball milling of the starting electrically conductive material also affects the electrical conductivity of the final lacquer layer 13. The hner the particle size of the electrically conductive particulate filler material 14, the lower will be the electrical conductivity of the lacquer `film layer 13. In general, however, typical ball milling times for the filling material 14 may be anywhere from 2 to 32 hours or so in a ball mill, with rotational speeds around 100 to 150 r.p.m., when milling the filling material while suspended in a liquid vehicle which is a solvent for the organic plastic of the matrix material 15. Milling time depends on the size of the mill, the type and number of mill stones, and the degree of loading of the mill jar, according to principles well known to those skilled in the art of milling. The suspending medium preferably employed for the filler ma- -terial 14 is the same solvent as that employed for the organic plastic matrix 15 of the conductive lacquer layer 13. If the milling is performed while the filling material 14 is in a dry state, i.e unsuspended in a liquid vehicle, then a milling time of about 30 minutes or so is ordinarily suflicient.

Because of the inherent photoconductivity of commercial grade indium oxide, the use thereof as the electrically conductive filler material 14 in the conductive lacquer layer 13 of an electroluminescent lamp 1 ordinarily (i.e., without any special compensating treatment of the indium oxide) will result in non-uniformity or variation in the brightness of such lamps, depending on the differences in the surrounding light conditions to which the lamps have been exposed immediately prior to their energization or starting in operation. Thus, lamps that have rested in the dark just before placing them in operation are dimmer than those that have been exposed to light. This undesirable effect of the photoconductivity of the indium oxide filler material on the brightness of the lamp 1 may be overcome by tiring the indium oxide powder in air, for a period of around one hour or so at an elevated temperature of around 1000 C., before its incorporation into the conductive lacquer 13. Such a firing of the indium oxide powder largely removes the photoconductivity thereof, i.e., renders its electrical conductivity substantially insensitive to incident light radiation, and also reduces its dark resistivity by about orders of magnitude. The firing of the indium oxide for such purpose may be performed either before or after the milling thereof to reduce its particle size. However, because of the added processing steps which would be required (i.e., filtering, drying and sieving) where such firing of the indium oxide is performed after the milling thereof, it is preferable to carry out the firing of the indium oxide before the milling thereu of since little improvement in elimination of photoconductivity is realized by tiring the indium oxide after the milling thereof.

After the milling of the electrically conductive iiller material 14 to reduce the particle size thereof as described above, the milled slurry of filler material (where the wet mill-ing procedure is employed for the filler material), or alternatively the dry filler material (where a dry milling procedure is followed), is then added to the proper amount of a solution of the organic plastic matrix material 15 in a suitable solvent, and the resulting Imixture is then thoroughly mixed in a` high speed mixer, preferably followed by a slow rolling of the lacquer in a container to remove bubbles from the mix. The resulting suspension is then ready for use as a conductive lacquer coating composition to form an electrically con- 12 ductive lacquer film layer or electrode 13 of the electroluminescent lamp 1. The solvent employed for the organic plastic matrix material 15 of the electrically conductive lacquer film 13 may either be of the so-called attacking type which will soften the underlayers (for example, the phosphor and insulating layers 8 and 11) on which the conductive lacquer is to be coated, or it may be of the non-attacking type which will not soften the underlayers. The use of a solvent of the attacking type is of advantage in that the solvent softening of the underlayer, eg., the lphosphor layer 8, permits an intermingling of the conductive lacquer film layer 13 with the underlayer which inherently produces a good bond therebetween. On the other hand, the use of a solvent of the non-attacking type, 'while obviating the danger of the conductive particles 14 penetrating the underlayer or phosphor layer 8 and thu-s producing electrical shorts in the finished lamp, nevertheless may be of some disadvantage in that it would not produce a sufficiently good bond between the conductive lacquer film layer 13 and the underlayer or phosphor layer 8 to prevent the separation or delamination thereof in the finished lamp 1. However, because of the stronger bond which it Aproduces between the conductive lacquer lilm layer 13 and the phosphor layer 8, it is preferred to employ a solvent of the attacking type which softens the phosphor layer 8. By coating and drying the conductive lacquer quickly on the phosphor layer 8, there is little time and opportunity for solvent penetration of the phosphor layer such as to cause much softening thereof. Any suitable solvent may be employed for the ofrganic plastic matrix material 15, the choice of solvent depending on the nature of the organic plastic employed. In accordance with well known practice, however, it is preferable to employ a mixture of solvents having differing rates of vaporization in order to avoid the formation of bubbles or pores in the final set plastic. In the case of the preferred cyanoethyl cellulose-cyanotheyl phthalate plastic matrix material 15, the preferred solvent therefor is a mixture (hereafter referred to as trisolvent) of aro-und equal parts, by volume, of acetone, methyl ethyl ketone and dimethyl formamide.

By way of example, and not of limitation, following are specific examples of preferred formulations and manners of preparing electrically conductive lacquers according to the invention, both of the weakly conductive and highly conductive types as referred to hereinabove for use in forming the electrically conductive lacquer film layer 13 of an electroluminescent cell or lamp 1 comprising our invention:

EXAMPLE 1 (Weakly conductive lacquer) Electrically conductive glass paper prepared in the manner described hereinabove and in lthe previously mentioned Jaffe Patents 2,774,004 and 2,849,339, and consisting of fine glass brils or microfibers compacted into paper-like sheet material and coated with the electrically conductive heat-decomposition product of indium basic trifluoroacetate (believed to be an indium oxide of poorly developed crystallinity) is placed in a ball mill with small (1/2") balls, and milled dry for one-half hour, The resultant powder is passed through a 200 mesh screen, and cornes out a dull blue-gray in color. The powder' is placed in an open quartz dish and fired 5 to 20 minutes at 500 to 700 C. (preferably 10 minutes at around 600 C), whereupon the body color of the powder comes a light yellow which is of lighter color than the original dull blue-gray colored powder. The lighter body color of the conductive glass powder results in less absorption of light passing through the layer Y13 of weakly conductive lacquer ilrns; hence, a brighter lamp is obtained. Two parts by weight of the electrically conductive glass powder are thoroughly mixed with one part by weight of cyanoethyl phthalate plasticizer, and then 13 with ten parts by weight of a by weight solution of cyanoethyl cellulose in trisolvent, to form a coating suspension of the following composition:

EXAMPLE 2 (Weakly conductive lacquer) An electrically conductive lacquer of a type employing indium oxide as the electrically conductive particulate ller material 14 may consist of the following composition:

To remove the photoconductivity from the indium oxide, it is lirst tired in `alundum boats in a quartz tube furnace. The indium oxide is put into the furnace at room temperature, the temperature then raised to around 1000o iC. during a period of about forty minutes, and held at this elevated temperature for a period of about one hour. The furnace is then cooled to less than 300 C. during a period of about two hours before the indium oxide is removed from the furnace.

The formula proportions of indium oxide and cyanoethyl phthalate, together with 45 grams of the trisolvent, are put into a one quart mill jar with fifty 1%@ inch by 1%6 inch Burundum cylinders and the mix then milled for approximately sixteen hours in the mill jar, with the jar rotating at a speed of about 110 r.p.m.

The exact formula amount of dry cyanoethyl cellulose is weighted into a jar and enough trisolvent added to make a 12% Iby weight 4solution (7.34 grams trisolvent per gram of cyanoethyl cellulose). The use of dry cyanoethyl cellulose insures that exactly the right amount of cyanoethyl cellulose will be present in the final lacquer composition, since the electrical conductivity of the lacquer can be changed considerably by small shifts in the indium oxide loading. This could accidentally occur if a bulk cyanoethyl cellulose solution were used which had lost some of its original solvent content or which originally had slightly more than the labeled concentration of cyanoethyl cellulose. Use of such a solution having a higher concentration of cyanoethyl cellulose would therefore introduce more -cyanoethyl cellulose into the iinal lacquer composition than the formula proportion, thus lowering the actual induim oxide concentration in the final lacquer composition.

The milled indium oxide and is poured directly into the jar containing the cyanoethyl cellulose solution. Two portions of triso-lvent totaling 41 grams is then used to rinse the mill jar of the remaining milled indium oxide and cyanoethyl phthalate mix. The combined indium oxide, cyanoethyl phthalate and cyanoethyl cellulose mix is then stirred with a spatula and rolled for about an hour in the mill jar. To prevent the presence of any undispersed agglomerates of indium oxide in the mix, it is placed in a one quart high speed electric mixer and mixed therein, this operation producing a very good dispersion ofthe indium oxide powder in the suspending vehicle. The viscosity lof the resulting weakly conductive lacquer composition is about 30 poises, and the cyanoethyl phthalate mix 14?; mix produces an electrically conductive lacquer having 'a specific impedance of the order of 1 megohm-cm., for 60 cycle current at a current density of 1 amp/cm?.

EXAMPLE 3 (Highly conductive lacquer) A strongly conductive lacquer of the type employing indium oxide as the electrically conductive particulate filler material 14 may consist of the following composition prepared as described hereinbelow:

Wet, grams Dried, vol.

percent Indium oxide 18. 68 24. 2 C yanocth yl cellulose. 5. U6 30. 0 Cyanoethyl phthalate.-. 4. 79 37. 8 Trisolvcnt 71 The formula proportions of cyanoethyl phthalate and indium oxide, tired as described above in Example 2, are placed, together with approximately 19 grams of trisolvent, in a one-half pint mill jar with twelve 1%6 inch `by 1%6 inch Burundum cylinders, and the mixture then milled for approximately eight hours, with a jar rotational speed of about r.p.m. In this case, the milling of the indium oxide powder is carried out for approximately only half the length of time (i.e., only eight hours) as in the case of the weakly conductive lacquer of Example 2 where the milling of the indium oxide powder is continued for sixteen hours. The lesser milling time results in a larger particle size for the indium oxide whi-ch, together with the increased volume loading thereof in the final lacquer composition, accounts for the increased electrical conductivity thereof. The milled indium oxide and cyanoethyl phthalate mix is then added to the formula proportion of cyanoethyl cellulose, in a 12% solution thereof in trisolvent, and mixed therewith in substantially the same manner as in Example 2 above, the jar containing the milled indium oxide and cyanoethyl phthalate mix being rinsed with approximately 15 grams of trisolvent to assure removal of all the indium oxide powder and cyanoethyl phthalate therefrom. The viscosity of the resulting conductive lacquer mix is around 30 poises, and the mix produces an electrically conductive lacquer having a specific impedance of around 5 to 8 ohm-cm. for 60 -cycle current at a current density of 1 amp/cm2.

EXAMPLE 4 (H glzly conductive lacquer) A highly conductive lacquer of the type employing crushed electrically conductive glass paper as the conductive particulate filler material 14 may consist of the following composition.

Electrically conductive glass paper, the same as employed in Example 1, is lirst tumbled with wood blocks in a tumbling barrel to crush it into a coarse conductive glass powder. The formula proportion of such conductive glass powder, together with approximately 25 grams of methyl ethyl ketone, are than placed in a one-half pint mill jar, with twelve 1% in-ch by 3A inch Burundum cylinders, and the mix then milled for approximately two hours at a mill rotational speed of around 290 r.p.m. To this milled mix is added a suspending vehicle prepared by rolling together, until mixed, the formula proportions of cyanoethyl sucrose, cyanoethyl cellulose and nitromethane. The combined ingredients are thoroughly intermixed by rolling the mill jar containing them for a period of approximately three and one-half hours at a mill ro- `phor layer 8 thereof, when the lamp is operated on a 120 volt 60 cycle alternating current.

In fabricating an electroluminescent lamp 1 of the type illustrated in FIG. 1 wherein the light-transmitting electrode is constituted solely of a layer 13 of a highly conductive lacquer film, the said lacquer layer may be formed either by applying a coating of a highly conductive lacquer suspension such as described hereinabove directly onto the phosphor layer 8, followed by a drying of the coating, or by applying such a lacquer coating onto a temporary release sheet of a suitable organic plastic, such as that known .as Mylar for instance, and then overcoating the dried lacquer layer with the phosphor layer 8 and the insulating or barrier layer 11. After thorough drying of the layers 8 and 11, the composite sheet composed of the highly conductive lacquer lm layer 13, the phosphor layer 8 and the insulating layer 11 may be lifted or peeled off the plastic release sheet and then laminated together with the other component elements of t-he electroluminescent lamp 1. The opaque back electrode 9 in such case may be conveniently constituted either of vacuum-evaporated aluminum or a silk screen printed commercial silver paint although, if desired, a sheet of aluminum foil can be laminated -to the insulating layer 11 to serve as the back electrode. The preferred manner, however, of forming the electrode layer 13 in an electroluminescent lamp 1 of the type illustrated in FIG. l is by applying a coating of the highly conductive lacquer suspension onto the desiccant sheet 12 of nylon or other similar hydrophilic organic plastic material which, on drying of the coating to form the conductive lacquer film electrode layer 13, is then laminated together with the other component elements of the electroluminescent lamp 1, with the conductive lacquer coated side 13 next to and in contact with the phosphor layer 8 of the lamp. In each of the methods described above, the coatings 13 of conductive lacquer may be conveniently applied to the respective substrates involved in each case by means of a conventional commercial type doctor blade coating device, and the thickness of the applied coating of the conductive lacquer suspension will generally be of the order of around 1 to 2 mils or so, where the coating suspension employed has a solids-to-solvent ratio of around 10 to 15%, in order to produce final dried conductive lacquer layers 13 within the desired thickness range of around 0.1 to 0.6 mil, corresponding to approximately 21/2 to 15 microns.

In the fabrication of an electroluminescent lamp 1 of the type illustrated in FIG. 2 wherein the light-transmitting electrode 16 is of composite character comprised of a light-transmitting electrically conductive web 17 of reticulate character, such as metal screening or electrically conductive paper, together with a layer 13 of light-transmitting electrically conductive lacquer film disposed between the phosphor layer 8 and the electrically conductive reticulate web 17 to serve as a conductivity aid therefor, the conductive lacquer film layer 13 in such case may be formed in the same manner as in the case of FIG. 1 by applying a coating of a weakly conductive lacquer suspension such as described hereinabove either directly onto the phosphor layer 8, yor onto a temporary release sheet of Mylar or otherr suitable organic plastic followed by the' overcoating of the weakly conductive lacquer layer 13 with the phosphor layer 8 and the insulating layer 11 after which the dried composite sheet composed of the conductive lacquer layer 13, phosphor layer 8 and insulating layer 11 is peeled off the release or backing sheet and then laminated together with the other elements of the electroluminescent lamp 1. In the case where the weakly conductive lacquer layer 13 is coated directly onto the phosphor layer 8, the electrically conductive paper layer or other electrically conductive reticulate web 17 may be either laminated to the dried conductive lacquer layer 13 during the final lamination of the lamp 1 or it may be first solvent-stuck to the conductive lacquer layer 13, as by moistening the conductive paper with a fine spray of trisolvent and the assembly then laminated into an electroluminescent lamp 1, or the electrically conductive paper `or other electrically conductive reticulate web 17 may be laid down onto the wet electrically conductive lacquer layer 13 as it is coated onto the phosphor layer 8. The electrically conductive paper layer 17, prior to its lamination to the dried conductive lacquer layer 13 of the electroluminescent lamp during the lamination of the latter, may be first solvent-stuck to the desiccant sheet 12 of nylon or other hydrophilic organic plastic, or else prelaminated thereto to form a composite electrically conductive plastic sheet material or prelaminate as described and claimed in copending U.S. Application Ser. No. 137,924 of Longfellow, filed Sept. 13, 1961 now U.S. Patent No. 3,226,272 and assigned to the assignee of the present invention. The electrically conductive lacquer film layer 13 may in such case be coated onto the conductive paper side 17 of such a prelaminate to form a composite electrode and desiccant sheet structure 18, as shown in FIG. 3, which is then laminated together with the other component elements of the electroluminescent lamp 1, with the conductive lacquer film side 13 next to the phosphor layer 8, to form the completed lamp.

The electrically conductive lacquer `film layer 13 according to the invention is of particular utility in an electroluminescent lamp of the type illustrated in FIG. 2 having a light-transmitting electrode comprised of an electrically conductive reticulate web of comparatively open mesh character such as fine wire cl-oth or fine perforated metal screen, or electrically conductive paper such as that referred to hereinabove and comprised of metallized glass microfibrils dispersed among, and randomly interlaced with and supported by translucent fibers of non-conducting material such as Orlon, nylon or rayon, for instance. Because of the comparatively open mesh structure which is required of such type reticulate web electrodes in order to have acceptable optical transmission for use in an electroluminescent lamp, the light produced by the phosphor layer 8 of the lamp appears only at the rims of the openings in the metal cloth or wire screen, or at the rims of the areas between adjacent ones of the metallized glass fibrils in the case of the conductive paper, the center of these openings or areas beingy dark due t0 insufficient electric field at that region. Also, in the case of metal cloth or wire screen type electrodes, the wires of which they are made act to obscure the light generated in those portions of the phosphor layer 8 which lie directly below the wires. For these reasons, electroluminescent lamps employing such metal grid type electrodes or conductive metallized glass fibril type paper electrodes such as described above, are characterized by a very dim and grainy appearing lighting effect. In addition, the metallized glass fibril type paper electrodes tend to fail and turn dark in a patchy manner, presumably due to the relatively nonuniform distribution of the electrically conductive fibrils in such type conductive paper which then causes burn-out to occur when too large a current load is carried by a single conductive fibrils connecting adjacent areas more dense with conductive brils. With such type conductive paper electrodes also, not all the conductive fibrils thereof lie directly on or in Contact with the phosphor layer 8 of the lamp, since the supporting non-conductive fibrils in the paper act to hold some of the conductive fibrils away from the phosphor layer,

causing a voltage drop in series. The provision, however, in such -a lamp of a layer 13 of a weakly conductive lacquer according to this invention between the comparatively open mesh or open structure type reticulate electrode 17 and the phosphor layer 8 of the lamp acts as a conductivity aid to not only spread the electrical field laterally across the surface of the retioulate electrode 17 so as to bridge the openings therein, i.e., the open areas between the adjacent conductive wires or fibrils thereof, but to also bring the electrical field down to the surface of the phosphor layer 8. The resulting electroluminescent lamp is not only characterized by a uniformly smooth (i.e., non-grainy) lighted `appearance of appreciably increased (i.e., greater than 100%) brightness comparable to that of the brightest electroluminescent lamps commercially available at present, but it is also markedly free of any tendency to fail and turn dark in a patchy manner caused by burn-outs in the paper elecrode of the lamp.

The electrically conductive lacquer according to the invention may yalso be of utility, as a conductive layer 13 in an electroluminescent lamp 1, in promoting intimate electrical contact between the relatively rough surface of the phosphor layer 8 and a smoother and fairly rigid transparent electrically conductive electrode layer 17 such as thin flexible glass sheet, or mica sheet, or mica paper, which have been rendered electrically conductive in a suitable manner as, for example, by the indium basic triii-uoroacetate process referred to hereinabove, or by heat treatment in the presence of the vapors of tin or indium chlorides.

Because of the ruggedness an-d non-fragile character of electrically conductive lacquer electrodes according to the invention, with resulting freedom from breakage dur-ing handling and cutting, and also because of their adaptability to formation by simple machine coating methods or by printing, the use of such a conductive lacquer solely by itself `as `the l-ight-transmitting electrode of -a flexible type electroluminescent lamp or similar device, or as one of the component elements of a composite type lighttransmiting electrode I'116 for such a device, results in a substantial manufacturing cost reduction amounting to the order of from five to ten fold over the prior type electrically conducting glass paper electrodes employed for such purpose. Moreover, and particularly in the case of the smaller size electroluminescent lamps, this cost saving advantage is realized with substantially no loss in brightness of the resulting lamp las compared to that obtainable from lamps equipped with the prior type fragile conductive glass paper electrode, lamps according to the invention consistently exhibiting brightnesses substantially equal to, and in some cases even higher than that of such prior type electroluminescent lamps. Such comparable brightnight levels are, of course, due in part to the high lighttransmission characteristic of the conductive lacquer electrode 13 according to the invention. The use, moreover, of a conductive lacquer layer 13 as `a conductivity aid for a reticulate web type electrode element 17 of an electroluminescent lamp, such as the metallized glass fibril type paper electrode referred to hereinabove, not only eliminates the grainy lighted appearance otherwise exhibited by such type lamps so that they present instead of uniformly smooth lighted appearance, but it also prevents the occurrence of localized burn-outs or failures in the electrode such as produce dark spots in the lighted appearance of the lamp.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the lart without departing from the invention. The appended claims are therefore intended `to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A light-transmitting electrically conductive lacquer consisting essenti-ally of a substantially transparent organic plastic having uniformly dispersed therein light-transmitting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles being present in said lacquer in an amount of at least 10% by volume and said lacquer having -a light transmittance of at least 60% at a film thickness of approximately 15 microns.

2. An electrode for an electroluminescent device comprising a sheet of transparent hydrophilic thermoplastic material having on Aa face thereof a thin layer of a lighttransmitting electrically conductive lacquer consisting essentially of a substantially transparent organic plastic having uniformly dispersed therein light-transmitting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles being present in said lacquer in an amount of at least 10% by volume and said lacquer having a light transmittance of at least 60% at a film thickness of approximately 15 microns.

3. An electroluminescent device comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers, one of said electrode layers being light-transmissive and being constituted solely by a film of a light-transmitting electrically conductive l-acquer consisting essentially of a substantially transparent organic plastic having uniformly dispersed therein light-transmitting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles being present in said lacquer in an amount of at least 10% by volume and said lacquer having a lighttransmittance of at least 60% at a film thickness of approximately 15 microns. i

4. An electroluminescent device comprising a layer of electroluminescent phosphor sandwiched between a pair of parallel electrode layers, one of said electrode layers being light-transmissive and comprising a substantially transparent organic plastic having uniformly dispersed therein -a light-transmitting electrically conductive particulate material the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive particulate material being present in said one electrode layer in an amount of at least 10% by volume and said one electrode layer having a light-transmittance of at least 60% at a film thickness of approximately 15 microns, said conductive particulate material being comprised of fibrous particles of glass coated with indium oxide and having a length substantially less than the distance between the said electrode layers.

5. An electroluminescent device comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers, one of said electrode layers being light-transmissive and comprising a substantially transparent organic plastic having uniformly dispersed therein light-transmitting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles having a maximum particle size of approximately 10 microns and being present in said one electrode layer in an amount of at least 10% by volume and said one electrode layer having a lighttransmittance of at least 60% at a film thickness of approximately 15 microns.

6. A light-transmitting electrically conductive lacquer consisting essentially of a substantially transparent organic plastic having uniformly dispersed therein light-transmitting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles being present in said lacquer in an amount of approximately 10 to 50% by volume.

7. A light-transmitting electrically conductive lacquer consisting essentially of a substantially transparent organic plastic having uniformly dispersed therein light-transmitting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles being present in said lacquer in an amount of at least'10% by volume and said lacquer having la lighttransmittance of at least 60% at a lm thickness within the range of approximately 0.1 to 0.6 mil.

8. An electroluminescent device comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers, one of said electrode layers being light-transmissive and being constituted solely by a lm of a light-transmitting electrically conductive lacquer consisting essentially of a substantially transparent organic plastic having uniformly dispersed therein lighttransmit ting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles being present in said lacquer in an amount of at least 10% by volume and said lacquer having a lighttransmittance of at least 60% at a lm thickness within the range of approximately 0.1 to 0.6 mil.

9. An electroluminescent device comprising a layer of electroluminescent phosphor sandwiched between a pair of electrode layers, one of said electrode layers being light-transmissive and comprising a light-transmitting electrically conductive lacquer consisting essentially of a substantially transparent organic plastic having uniformly dispersed therein light-transmitting electrically conductive particles of indium oxide the electrical conductivity of which is substantially insensitive to incident light radiation, said conductive indium oxide particles having a maximum particle size of approximately 10 microns and being present in said lacquer in an amount of at least 10% by volume and ranging up to approximately 50% by volume.

References Cited by 'the Examiner UNITED STATES PATENTS 2,918,594 12/1959 Fridrich 313-108 3,148,107 8/1964 Selke 162-138 3,161,797 12/1964 Butler et a1 313-108 JAMES W. LAWRENCE, Primary Examiner. R. JUDD, Assistant Examinez'. 

1. A LIGHT-TRANSMITTING ELECTRICALLY CONDUCTIVE LACQUER CONSISTING ESSENTIALLY OF A SUBSTANTIALLY TRANSPARENT ORGANIC PLASTIC HAVING UNIFORMLY DISPERSED THEREIN LIGHT-TRANSMITTING ELECTRICALLY CONDUCTIVE PARTICLES OF INDIUM OXIDE THE ELECTRICAL CONDUCTIVITY OF WHICH IS SUBSTANTIALLY INSENSITIVE TO INCIDENT LIGHT RADIATION, SAID CONDUCTIVE INDIUM OXIDE PARTICLES BEING PRESENT IN SAID LACQUER HAVING A LIGHT TRANSMITTANCE OF AT LEAST 60% AT A FILM THICKNESS OF APPROXIMATELY 15 MICRONS.
 3. AN ELECTROLUMINESCENT DEVICE COMPRISING A LAYER OF ELECTROLUMINESCENT PHOSPHOR SANDWICHED BETWEEN A PAIR OF ELECTRODE LAYERS, ONE OF SAID ELECTRODE LAYERS BEING LIGHT-TRANSMISSIVE AND BEING CONSTITUTED SOLELY BY A FILM OF A LIGHT-TRANSMITTING ELECTRICALLY CONDUCTIVE LACQUER CONSISTING ESSENTIALLY OF A SUBSTANTIALLY TRANSPARENT ORGANIC PLASTIC HAVING UNIFORMLY DISPERSED THEREIN LIGHT-TRANSMITTING ELECTRICALLY CONDUCTIVE PARTICLES OF INDIUM OXIDE THE ELECTRICAL CONDUCTIVITY OF WHICH IS SUBSTANTIALLY INSENSITIVE TO INCIDENT LIGHT RADIATION, SAID CONDUCTIVE INDIUM OXIDE PARTICLES BEING PRESENT IN SAID LACQUER IN AN AMOUNT OF AT LEAST 10% BY VOLUME AND SAID LACQUER HAVING A LIGHTTRANSMITTANCE OF AT LEAST 60% AT A FILM THICKNESS OF APPROXIMATELY 15 MICRONS. 